Network switches inspect fields in the headers of Ethernet packets in order to determine what action to perform on the packet. Actions may include sending the packet to a specific output port and a specific queue for that port, multicasting or broadcasting the packet to several ports, sending the packet to a network controller so it can determine the packet's outcome, or dropping the packet by not sending it to any destination. Packets have headers representing several levels of the 7 layer OSI (Open Systems Interconnection) model defining packet transport. Typically, these headers follow one another in increasing layer number of the OSI model. As an example, a packet may begin with a MAC (Media Access Control) header at level 2, followed by an IP (Internet Protocol) header at level 3, followed by a TCP (Transmission Control Protocol) header at level 4. Each of these headers internally has a number of individual fields which may be inspected by the switch. At the level 2 MAC layer, the MAC header contains MAC source and destination addresses, along with an ethertype. An IP header at layer 3 contains IP source and destinations and an IP protocal, among other fields. A TCP header at level 4 contains TCP source and destination port numbers, among other fields. All these fields may be inspected by the switch, and may be modified in the course of determining the disposition of the packet.